Nitrogen is an important plant nutrient. In addition to phosphorous, potassium, and other nutrients, nitrogen is needed to support the growth and development of plant life. Some plants, such as legumes, through a symbiotic relationship with Rhizobium bacteria take up elemental nitrogen from the atmosphere and fix this nitrogen into the soil. However, most plants grown to produce human and animal food require the use of nitrogen fertilizer in order to sustain their agricultural production.
The most widely used and agriculturally important high-analysis nitrogen fertilizer is urea, CO(NH.sub.2).sub.2. While most of the urea currently produced is used as a fertilizer in its granular form, urea-based fluid fertilizers are also well known. As used herein, the term "fluid fertilizers" encompasses liquid fertilizers, i.e. aqueous solutions of fertilizers, and suspension fertilizers, i.e. fertilizer compositions which in addition to water and water-soluble components also contain insoluble components kept in suspension by a suspending agent, such as clay. Suspension fertilizers are excellent carriers for pesticides and micronutrients.
The most commonly known urea-based liquid fertilizer is an aqueous solution of urea and ammonium nitrate, referred to in the fertilizer trade as a UAN solution. The corresponding urea-based suspension fertilizer is an aqueous solution of urea and ammonium nitrate also containing a suspending agent, such as clay. These fluid fertilizers are used on a variety of crops, such as corn, wheat and rice. When applied to moist soil, the urea content of the fluid fertilizer becomes a source of ammonia as a result of hydrolysis catalyzed by urease, an enzyme produced by numerous fungi and bacteria. The reaction may be written as follows: ##STR1##
The ammonia formed as shown above undergoes very rapid hydrolysis to form ammonium ions in accordance with the following equilibrium: EQU NH.sub.3 +H.sub.2 O.revreaction.NH.sub.4.sup.+ +OH.sup.-
In most soils, the ammonium formed through hydrolysis as shown above, as well as the ammonium originally supplied as ammonium nitrate are readily converted to nitrate via a sequence of bacterial oxidation reactions; the overall oxidation reaction may be written as follows: EQU NH.sub.4.sup.+ +2O.sub.2 .fwdarw.NO.sub.3.sup.- +H.sub.2 O+2H.sup.+
and is commonly referred to as "nitrification".
Both, ammonium nitrogen derived through the hydrolysis of urea or supplied as an ammonium compound as well as nitrate nitrogen resulting from the oxidation of ammonia or supplied as a nitrate compound may be assimilated directly by the plant. Thus, the urease-catalyzed hydrolysis of urea and the bacterial oxidation of ammonium are two key steps in the vital transformation of urea nitrogen first into ammonium nitrogen and then into nitrate nitrogen, both of which function in soils as nitrogen nutrients.
The major problems associated with the use of urea-containing fluid fertilizers, such as UAN, as a source of nitrogen nutrient to support the growth of crop plants relate to the fact that the time frame for the catalytic hydrolysis of urea to ammonia and for the subsequent nitrification of ammonium does not coincide with the ongoing demand for nitrogen by the root system of the plants. More specifically, the catalytic hydrolysis of urea and the subsequent nitrification of the ammonium ions proceed relatively rapidly, i.e. within 2 to 20 days, as compared to the 50 to 200 day growing seasons for typical crop plants. Since both ammonia and nitrate can be lost from the soil by various mechanisms before being assimilated by the plant, the premature conversion of urea into ammonium and nitrate nitrogen contributes to the low (40%) efficiency with which crop plants utilize fertilizer nitrogen. Examples of mechanisms by which nitrogen can be lost from the soil include loss of ammonia through volatilization to the atmosphere and loss of nitrate through leaching to the subsoil by rainwater and/or through denitrification, i.e. bacterial conversion of nitrate to elemental nitrogen. Another drawback related to rapid hydrolysis of urea is the potential for excessive accumulation of ammonia in the soil shortly after seeding which may result in damage to germinating seedlings and young plants.
Prior art offers two approaches to make nutrient nitrogen derived from fluid urea-containing fertilizers available to root systems of plants throughout their growing season: (1) multiple fertilizer applications, and (2) the incorporation of urease inhibitors or nitrification inhibitors into the fertilizer formulation. There are certain limitations and drawbacks associated with each of these approaches advocated by prior art.
The first approach involves the use of multiple fertilizer applications during the course of a crop growth season. Such multiple fertilizer applications can provide adequate nitrogen to meet the demands the growing plants, but they do so at the expense of higher fertilizer costs, higher fertilizer application costs, and of an adverse environmental impact associated with the loss of nitrate through leaching to the subsoil.
The second approach toward improving the availability of nitrogen to the root system of plants over an extended period of time entails the incorporation of a urease inhibitor or of a nitrification inhibitor into urea-containing fertilizers. Urease inhibitors are compounds capable of inhibiting the catalytic activity of the urease enzyme upon urea in moist soil. Among the most effective urease inhibitors are the phosphoric triamide compounds disclosed in U.S. Pat. No. 4,530,714. An example of an effective urease inhibitor disclosed in the '714 patent is N-(n-butyl)thiophosphoric triamide, which will be referred to herein as NBPT. When incorporated into a fluid urea-containing fertilizer, NBPT reduces the rate at which urea is hydrolyzed in the soil to ammonia. The benefits realized as a result of the delayed urea hydrolysis include the following: (1) nutrient nitrogen is available to the plant over a longer period of time, (2) excessive build up of ammonia in the soil following the application of the urea-containing fertilizer is avoided, (3) the potential for nitrogen loss through ammonia volatilization is reduced, (4) the potential for damage by high levels of ammonia to seedlings and young plants is reduced, (5) plant uptake of nitrogen is increased, and (6) an increase in crop yields is attained. While NBPT does not directly influence the rate of ammonium nitrification, it does control the levels of ammonium which are subject to the nitrification process and thereby indirectly controls the levels of nitrate nitrogen generated in the soil.
NBPT has not been commercially used heretofore as an additive in fluid urea-containing fertilizers, presumably because of the lack of a suitable method for the preparation of such fluid fertilizers stemming from certain physical and chemical characteristics of industrial grade NBPT which render this material difficult to handle. Industrial grade NBPT is a waxy, sticky, heat-sensitive and water-sensitive material. Consequently, the material is susceptible to decomposition during storage and methodology for metering NBPT into production equipment has been heretofore unavailable.
The availability of nitrate nitrogen to plants over an extended period of time can also be enhanced through the incorporation of nitrification inhibitors into urea-containing fertilizers. Nitrification inhibitors are compounds capable of inhibiting the bacterial oxidation of ammonium to nitrate in the soil. Among the most effective nitrification inhibitors is dicyandiamide, also referred to as DCD. A fluid urea-containing fertilizer formulation containing DCD is disclosed in U.S. Pat. No. 5,024,689. While DCD does not affect the rate at which urea is hydrolyzed to ammonia in the soil, it significantly reduces the rate at which ammonium is oxidized to nitrate. The benefits realized as a result of the delayed nitrification process include the following: (1) nutrient nitrogen is available to the plant over a longer period of time than is the case in the absence of DCD, (2) the potential for loss of nitrate nitrogen through denitrification and/or leaching is reduced, (3) plant uptake of nitrogen is increased, and (4) crop yields are increased. However, the improvement in the performance of fluid fertilizers containing urea and DCD which can be attributed to the incorporation of DCD in these formulations is believed to be severely limited by the susceptibility of these formulations to urease-catalyzed hydrolysis following application of the fertilizer to the soil. This may result in relatively high ammonia losses through volatilization and in ammonia damage to seedlings and young plants.
In addition to the foregoing, U. S. Pat. Nos. 4,517,003; 4,517,004; 4,932,992; and 4,954,156 make reference to various compounds which are capable of inhibiting both the urease-catalyzed hydrolysis of urea and the oxidation of ammonium to nitrogen. None of these, however, have found commercial acceptance in the fertilizer industry as additives capable of improving the performance of urea-containing fluid fertilizers in terms of their ability to enhance crop yields.
Accordingly, it is an object of this invention to provide a fluid urea-containing fertilizer formulation which offers an effective alternative to the high amounts of urea-containing nitrogen fertilizer that are currently used to assure that crop yields are not limited by the availability of nitrogen as a plant nutrient.
It is a further object of this invention to increase nitrogen uptake efficiency of urea-containing fluid fertilizers.
It is another object of this invention to provide a urea-based fluid fertilizer formulation the performance of which is characterized by relatively low ammonia volatilization losses, low losses of nitrate nitrogen through denitrification and/or leaching, and substantially enhanced crop yields.
It is still another object of this invention to provide a method for the production of the urea-containing fluid fertilizer formulations disclosed hereinbelow.